The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Optical recording devices are used to record information, such as music, movies, pictures, data, etc., on recordable media. Examples of recordable media are compact discs (CDs), digital versatile/video discs (DVDs), high density/high definition DVDs and Blu-ray Discs (BDs). In order to record such information, a recording device tracks the location of a laser beam on the recordable media.
Referring to FIG. 1, a side close-up view of a recordable media 10 is shown. The recordable media 10 has lands 12 and grooves 14, which are formed on a recording layer 16 of a main substrate 18. The main substrate 18 may be adhered via an adhesive layer 20 to a dummy substrate 22, as shown. The lands 12 and grooves 14 refer to physical structures of the recording layer 16, which are adjacent each other, but have different associated depth. The grooves 14 have a larger associated depth than the lands 12. A sample land depth D1 and a sample groove depth D2 are shown. The depths are measured relative to and from a disk outer or entrance surface 24 and are equal to some fraction of optical wavelength of the laser beam.
The purpose of land/groove structures is to provide servo information for positioning of a laser beam spot on a disc. The land/groove structures provide reflected beam signal modulation that is detected for tracking purposes. The lands and grooves are also often created with a small amount of waviness at a pre-determined characteristic frequency, referred to as “wobble”. This allows clock frequency extraction during a recording process.
Standards, such as DVD+/−R and DVD+/−RW, require recording only over grooves. An alternative standard, referred to as DVD-RAM, requires recording over both land and groove structures. In DVD+/−R and DVD+/−RW recording, the grooves and lands typically form a continuous spiral. In DVD-RAM recording, the land structures alternate with the groove structures to form a continuous spiral.
Referring to FIG. 2, a sample optical DVD drive system 50 is shown and includes a laser source 52, such as a laser diode, that provides a laser beam 54. The laser source 52 may be part of an optical read/write assembly (ORW) 56, sometimes referred to as an optical pick-up assembly. The ORW 56 includes a collimator lens 58, a polarizing beam splitter 60, a quarter wave plate 62, and an objective lens 64. The laser beam 54 is collimated by the collimator lens 58 and passed through the polarizing beam splitter 60. The laser beam 54 is received by the quarter wave plate 62 from the beam splitter 60 and is focused via the objective lens 64. The laser beam 54 may be radially displaced across tracks of the optical medium 68 through movement of the ORW 56 via a sled motor 66. The laser beam 54 is moved while the optical medium 68 is rotated about a spindle axis 69. The laser beam 54 is shaped and focused to form a spot over the land/groove structures of an optical storage medium 68 via lens actuators 70.
The light from the laser beam 54 is reflected off of the optical medium 68 and directed back into the ORW 56. The reflected light, represented by dashed line 72, is redirected by the beam splitter 60 and focused into a spot over a photo-detector integrated circuit (PDIC) 74 by an astigmatic focus lens 76. Although not shown, additional photo-detectors may be incorporated and used to detect other diffracted light beams not shown.
Referring now also to FIG. 3, a quad PDIC 100 is shown. The PDIC 100 has an array of photo-detector sensors A-D that receive a focused reflected laser beam, such as the reflected beam 72. The intensity of the reflected and focused laser beam spot on the PDIC 100 is not uniform. The distribution of that intensity depends on the position of the beam spot on a recordable medium relative to the land/groove structures. The lack of uniformity results in tracking error.
The tracking error is based on photo-detector output signals PA, PB, PC and PD of photo sensor elements A-D of the quad PDIC 100. The tracking error (TE) or TE signal is provided by equation 1.TE=(Pa+Pd)−(Pb+Pc)  (1)The TE signal is zero when the beam spot is fully over a groove, which is referred to as 100% on-track. The TE signal is also zero when the laser beam spot is half-way between tracks, which is referred to as 100% off-track.
Referring now also to FIG. 4, a curve 120 of TE signal variation as a function of beam spot position on a recordable medium is shown. The TE curve 120 represents TE signal amplitude versus radial direction positioning of a laser beam. The TE curve 120 is sinusoidal and has multiple zero-crossings 122, which represent different tracks. To maintain the beam spot on a track, the associated optical drive needs to lock onto the zero-crossing of a slope of the TE curve 120. In other words, the optical drive needs to detect the slope of the TE curve 120 when the TE signal is zero. However, additional information is needed when moving between tracks, especially when moving across several tracks. This is accomplished by monitoring the number of instances when the TE signal is equal to zero while radially moving the laser beam.
Misalignment can occur between the center of the recordable medium and the spindle axis of rotation, such as the spindle axis 69. This mis-alignment can cause “radial run-out”. For tracking purposes, when there is no radial run-out, it is sufficient to move the laser beam in one direction and count the number of zero-crossings of the TE signal. But typically, radial run-out occurs and causes the laser beam to cross one or more tracks. The radial run-out can alternate between radially inward and radially outward directions. This can occur when radial actuators and/or the associated sled motor are not being driven. As a result, simply counting zero-crossings of the TE signal does not provide the aggregate and appropriate number of track crossings in one direction.
To account for radial run-out, a second signal 130, referred to as a quad-sum (QSUM) signal, which is in quadrature to the TE signal, is generated and monitored. Referring to FIG. 5, a relationship between the TE signal and the QSUM signal are shown. The QSUM signal is also sinusoidal and is shifted 90° in phase relative to the TE signal. The QSUM signal is generated through the sum of the photo-detector output signals PA, PB, PC and PD as in equation 2.QSUM=PA+PB+PC+PD  (2)When the laser beam is positioned over a land region, the QSUM signal is at a maximum amplitude (a track boundary). When the laser beam is positioned over a groove region, the QSUM signal is at a minimum amplitude. The QSUM signal provides an accurate technique for counting track crossings. A track crossing counter is incremented or decremented at the zero-crossings 122 when the QSUM signal is at a maximum amplitude. Track crossings can also be defined as the moment when the QSUM signal is at a minimum amplitude, as denoted by dashed lines 132.
Unfortunately, for recordable optical media, the depth of modulation in the QSUM signal depends greatly on the media type and whether the media is written with user data or is a blank disc. The depth of modulation refers to the amplitude difference in the QSUM signal between on-track and off-track. The depth of modulation of the QSUM signal is shallow for blank recordable media. A shallow depth can render the QSUM signal virtually ineffective for track center location determination. Thus, it is difficult to perform an accurate seek over a blank recordable media due to the lack of a high contrast QSUM signal. Optical drive performance is degraded when a seek is inappropriately performed. The improperly performed seek is often detected after track following and reading of wobble information. This later detection forces another inaccurate seek when an attempt is initiated to move a read/write head to a target track position.